Heat the oven to 190C, (170C fan) Gas 5. Grease an ovenproof dish.
Heat the oil in a frying pan and gently cook the onion and the mushrooms until soft but not browned.
Add the spinach and ricotta and mix together.
Place 1tbsp of the spinach mixture at one end of a lasagne sheet and roll up to create a stuffed tube. Prepare all 8 sheets in this way.
Put the chopped tomatoes and garlic in the baking dish and place the cannelloni on top.
Make a cheese sauce by melting the butter in a pan, stir in the flour
and cook for 1 minute. Gradually add the milk, stirring well after each addition.
Add the Cheddar cheese and mustard powder and cook for 2 minutes until thickened.
Pour over the Cannelloni, sprinkle with grated Parmesan and cook for 30-35 minutes until golden brown. Serve with a crisp salad.
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